Stepping motors may be grouped into two general categories, that is, those having magnetically permeable rotors, i.e., reluctance rotors and those having permanent magnet rotors. Although the following description relates to the control of a permanent magnet motor, the circuitry will find similar application when used with variable reluctance motors. The windings of a stepping motor are usually mounted with their axis radially oriented around the rotor and fixed to the stator of the motor. Several windings are spaced about the stator and selectively energized to position the rotor in a predetermined angular position with respect to the windings. In a permanent magnet motor, the rotor moves so as to place a permanent magnet pole as close as possible to the opposite pole generated electromagnetically by an appropriate winding.
In a printing device of the type disclosed in U.S. Pat. No. 3,982,622, granted on Sept. 28, 1976, to J. A. Bellino et al., paper is fed across a print head by a platen rotatably driven by a stepping motor. The paper is advanced by the motor to the desired line position and once this position is attained, the paper must be held firmly in place during printing. The residual magnetic attraction between the rotor and stator of the motor in its de-energized condition is generally insufficient to hold the platen and paper firmly in place. To assure secure positioning of the platen during printing, a current is passed through the appropriate stator winding generating a magnetic field. Under such conditions the required current through the winding is considerably less than that necessary to produce rotation of the motor. Thus, the energy supplied to the motor during stationary operation is reduced to minimize power consumption and reduce motor heating. One method of attaining this end is to insert a resistance in series with the winding. However, such an arrangement dissipates considerable power in the resistor. Alternatively, the source voltage is periodically interrupted to reduce the average current flow through the winding. The rapid interruption of voltage to the inductive stator winding generates induced voltages which, if not clamped, may reach extremely high levels possibly destroying various components of the motor control circuit. During normal rotary operation such clamping is often provided by a zener diode which quickly dissipates the induced energy in the de-energized winding as described in U.S. Pat. No. 3,760,252, entitled "Damping Of A Step Servo Motor Using One Step Anticipation Logic" issued Sept. 18, 1973, to J. M. Berry. However, during stationary operation, when the winding current is chopped, such zener clamping is undesirable since a rather large amount of the energy supplied to the coil is dissipated by the zener diode. The following embodiment provides a circuit for quickly dissipating the induced energy in the stator winding of a stepping motor during normal rotary operation and for reducing this energy dissipation during stationary operation thereby raising the operational efficiency of the motor.